The Establishment Of Seamless Chart Datum

Measurement datum is the basis to which the surveys and spatial information are referenced. Nowadays three-dimensional coordinate system and its frame have been established with modern geodetic techniques, and are expressed on 2+1 dimension frame. Thereinto, the first two dimensions says coordinate frame, the third says vertical datum.Marine vertical datum is with prominent position in the modern marine surveying and mapping benchmark construction, and is with more and more important theoretical and realistic significance for hydrographic surveys and marine spatial information product production. Generally, local depth datum is used as reference datum in a sea area for hydrographic surveys. The ordinance datum is lowest normal low water in China. Chart datum is expressed by values of depth datum of discrete tide stations, and is extended to the entire area by the method of partition interpolation. Chart datum has obvious regional. The difference between chart datum of adjacent tide stations and National Height Datum 1985 is different, that depth datum is ladder-like and seaming.Surface fitting is probed to construct a seamless chart datum model which reflects the definition and continuity and gradual change of chart datum, including shepard, mobile surface, hardy function and B-spline function. For the study in the yellow sea and bohai sea waters, fitting precision of four methods is compared, and the problems that should pay attention to in the use are raised. B-spline function is used to construct models of lowest normal low water of two adjacent waters, then two models is spliced by polynomial fitting. Finally, separation model of the fitting datum relativing to the reference ellipsoid is constructed.